1. Field of the Invention
The present invention relates to a differential gear suitable for use in automotive vehicles, and more specifically to a small-sized differential gear provided with both differential limiting and locking functions,
2. Description of the Prior Art
An example of prior-art differential gears is disclosed in Japanese Published Examined (Kokoku) Application No. 60-580, for instance. In this prior-art differential gear, since a differential case is coupled to two vehicle wheel shafts by means of a dog clutch, the differential function thereof can be locked to facilitate extricating a vehicle from a slippery road.
In the prior-art differential gear as described above, however, although the differential locking function can be attained, it is impossible to attain the differential limiting function on the basis of a frictional force of the clutch, thus raising a problem in that vehicle steering is not stable at sharp corners.
To overcome this problem, certain differential gears for limiting the differential function are proposed, in which a rotative power transmitted from a differential case to a pinion shaft is further applied to a pressure ring via a cam to shift the pressure ring in the axial direction thereof so that a frictional force can be generated between the differential case and the pressure ring via a number of friction plates. An example of this arrangement is disclosed in U.S. Pat. No. 4,679,463. In this prior-art differential gear, there exists such an advantage that the differential function can be limited by a frictional force and further the differential limiting force increases with increasing torque inputted to the differential case. In this prior-art differential gear, however, since the differential function is limited only according to an input torque, it is impossible to lock the differential function when required, thus resulting in a problem in that it is difficult to allow the vehicle to be extricated from a slippery road.
On the other hand, although it may be possible to simply combine the two above-mentioned differential gears, another problem may occur in that the size of the differential gear is increased, in particular in the width (vehicle front and rear) direction of the differential gear.